The objective of the proposed investigation is to define the hormonal factors responsible for initiation and maintenance of spermatogenesis and to elucidate the influence of gonadotropins on androgen synthesis and secretion by the testes. Specifically we will address ourself to the following problems. Studies will be conducted to determine the hormonal requirements for gonocytes development. This will be done utilizing quantitative techniques of assessment of gonocytes numbers during development, radioautography of tritiated thymidine incorporation into the gonocytes under various hormonal conditions. The total hormonal milieu will be modified by the use of antiandorgens and antigonadotropins as well as surgical procedures to remove the fetal pituitary. The second question will be to elucidate the role of 5 alpha-reduced androgens and FSH in spermatogenesis. In this set of experiments a variety of animal preparations characterized by gonadotropin deficiency and testosterone deficiency will be treated with various 5 alpha-reduced androgens and specific segments of spermatogenesis will be analyzed. The study of the role of FSH in spermatogenesis will be approached in similar fashion. The second objective of this study, the differentiation of steroid biosynthetic pathways in the testes will be investigated from the viewpoint of the possible role of FSH in steroidogenesis, the possible role of 20 alpha-hydroxysteroid dehydrogenase in regulating of testicular function and the correlation of steroid biosynthetic activity in vitro and in vivo formation of androgens in testes of developing rats.